To improve the emission behavior of internal combustion engines, provisions are increasingly being made for fitting exhaust gas particle filters. The primary object of these filters is to capture the soot particles harmful to the environment. Latest proposals suggest catalytically coating the filtering channels of these exhaust gas particle filters, making it possible to neutralize further pollutants from the combustion. It is obvious that the soot particles captured will, in time, inevitably clog the filter: the flow resistance of the exhaust gas stream then increases extremely, which adversely affects the efficiency of the internal combustion engine. Countermeasures are aimed at eliminating the soot coating by combustion, through increasing the filter temperature permanently or for a short time.
However, in order that this combustion can take place at all, it must be ensured that the exhaust gases bring in sufficient oxygen during the combustion of the soot coating on the filter.
Therefore, in principle it is a question of on the one hand recirculating exhaust gas into the combustion air of the engine, to increase the exhaust gas temperature and thus the filter temperature for the purpose of regenerating the exhaust gas particle filter, and on the other hand of ensuring the minimum necessary or the optimum desired oxygen supply.
To regulate the oxygen content in regeneration of the exhaust gas particle filter, and consequently to control the throttle valve, a "lambda probe" is fitted between engine and exhaust gas particle filter as an oxygen sensor, the measuring signal of which is fed to a control system of the internal combustion engine, which acts in a suitable way on the fresh air supply and/or the quantity of fuel.
A "lambda probe", with a ZRO.sub.2 ceramic, suitable for measurement of the oxygen content in the exhaust of internal combustion engines relative to the oxygen content of the air has become known for example from the article by Hans-Martin Wiedenmann et al. "Heated Zirconia Oxygen Sensor for Stoichiometric and Lean Air-Fuel Ratios", SAE-Paper 840141, SAE-Congress, Detroit, February-March 1984.
In principle, it may be stated that the oxygen partial pressure in the exhaust gas varies however with the exhaust gas pressure. Now the pressure of the exhaust gas in the exhaust system of an internal combustion engine is by no means constant, but depends strongly on the degree of clogging of the exhaust gas particle filter and on the engine speed. In the case of supercharged internal combustion engines, the pressure fluctuations in the exhaust system are much greater, as the supercharging ratio concerned is also added to the said effects of engine speed and degree of clogging of the exhaust gas particle filter.
As far as a circuit of a supercharged internal combustion engine is concerned, this means that if the lambda probe is fitted in the high-pressure exhaust gas stream, the pressure prevailing there proves to be an inadmissible disturbance, because the: output signal of the lambda probe is pressure-dependent: all in all, the pressure of the exhaust gas in the exhaust system may fluctuate by a multiple of the air pressure. It goes without saying that, under such conditions, measurement of the percentage content of oxygen in the exhaust by means of the known lambda probe screwed directly into a wall of the exhaust system does not provide any useful results. If a remedy to this is sought, this requires a pressure correction or fitting of the lambda probe in a bypass part-stream of the exhaust system, the latter remedy preferably upstream of the exhaust gas particle filter if the circuit is provided with one.
However, pressure correction, which could eliminate the effect of the exhaust gas pressure on the measuring signal of the lambda probe, is conditional on using a pressure sensor and an electronic computer unit. But this is an elaborate solution, as the pressure sensor in the exhaust system has to be extremely corrosion-resistant.
The other precaution as well, namely fitting the lambda probe in a bypass part-stream in the exhaust system, proves to be an elaborate solution, both in terms of fitting the aid in a suitable place in the circuit and also in terms of the means used.